Transparent photodetector

ABSTRACT

The transparent photodetector includes a substrate; a waveguide on the substrate; a displaceable structure that can be displaced with respect to the substrate, the displaceable structure in proximity to the waveguide; and a silicon nanowire array suspended with respect to the substrate and mechanically linked to the displaceable structure, the silicon nanowire array comprising a plurality of silicon nanowires having piezoresistance. In operation, a light source propagating through the waveguide results in an optical force on the displaceable structure which further results in a strain on the nanowires to cause a change in electrical resistance of the nanowires. The substrate may be a semiconductor on insulator substrate.

BACKGROUND

The exemplary embodiments relate to sensing devices and, moreparticularly relate to photodetectors which can detect light with onlynegligible absorption of photons.

Photodetectors are devices that translate light intensity into anelectric signal. Photodetectors are widely used in optical communicationsystems from remote controls to fiber optic communication. Mostphotodetectors are semiconductor p-i-n junctions working on theprinciple of photon-induced carrier generation. Photons of energy largerthan the semiconductor bandgap are absorbed by the semiconductormaterial. The absorption generates electron-hole pairs that are thendetected by measuring the resulting current.

BRIEF SUMMARY

The various advantages and, purposes of the exemplary embodiments asdescribed above and hereafter are achieved by providing, according to afirst aspect of the exemplary embodiments, a transparent photodetector.The transparent photodetector includes a substrate; a waveguide on thesubstrate; a displaceable structure that can move with respect to thesubstrate and is in proximity to the waveguide; and a silicon nanowirearray suspended with respect to the substrate and mechanically linked tothe displaceable structure, the silicon nanowire array comprising aplurality of silicon nanowires having piezoresistance. In operation,light propagating in the waveguide results in an optical force on thedisplaceable structure which further results in a strain on thenanowires to cause a change in electrical resistance of the nanowires.

According to a second aspect of the exemplary embodiments, there isprovided a transparent photodetector. The transparent photodetectorincludes a silicon on insulator (SOI) substrate comprising asemiconductor on insulator (SOI) layer, an oxide layer, and a base; awaveguide formed in the SOI layer; a displaceable structure in proximityto the waveguide, the displaceable structure formed in the SOI layerwith the oxide layer removed from underneath the displaceable structureso that the displaceable structure is suspended with respect to the SOIsubstrate; and a silicon nanowire array in the SOI layer suspended withrespect to the base by the removal of the oxide layer underneath thesilicon nanowire array and mechanically linked to the displaceablestructure, the silicon nanowire array comprising a plurality of siliconnanowires having piezoresistance. In operation, a light sourcepropagating through the waveguide results in an optical force on thedisplaceable structure which further results in a strain on thenanowires to cause a change in electrical resistance of the nanowires.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

The features of the exemplary embodiments believed to be novel and theelements characteristic of the exemplary embodiments are set forth withparticularity in the appended claims. The Figures are for illustrationpurposes only and are not drawn to scale. The exemplary embodiments,both as to organization and method of operation, may best be understoodby reference to the detailed description which follows taken inconjunction with the accompanying drawings in which:

FIG. 1 is a top view of a first functional exemplary embodiment.

FIG. 2 is a top view of a second functional exemplary embodiment.

FIG. 3 is a top view of an implementation of a first exemplaryembodiment.

FIGS. 4A, 4B and 4C are cross sectional views of FIG. 3 along the lines4A-4A, 4B-4B and 4C-4C.

FIG. 5 is a top view of an implementation of a second exemplaryembodiment.

FIG. 6 is atop view of an implementation of a third exemplaryembodiment.

FIGS. 7A, 7B and 7C are cross sectional views of FIG. 6 along the lines7A-7A, 7B-7B and 7C-7C.

DETAILED DESCRIPTION

There is proposed a transparent photodetector in which there is a devicecapable of detecting light with negligible absorption of photons. Theproposed device does not rely on carrier generation in semiconductors.Rather, the proposed device relies on optical forces and the largepiezoresistance coefficient of silicon nanowires for detection of light.

The proposed transparent photodetector includes at least one waveguide,at least one displaceable structure in proximity to the waveguide and atleast one silicon nanowire array mechanically linked to the displaceablestructure. A silicon nanowire array means at least two nanowires.

Light propagating in the waveguide results in an optical force on thedisplaceable structure that is in proximity to the waveguide. Theoptical force may be attractive or repulsive. The applied optical forceresults in a strain on the nanowire array. The strain in the nanowirearray changes the resistance of the nanowires through the enhancedpiezoresistance effect in silicon nanowires. The change of resistancecan be detected by applying a voltage and measuring the output current.The output current will be related to the power of the optical signal inthe waveguide.

The piezoresistive effect describes the changing resistivity of asemiconductor due to applied mechanical strain. The piezoresistiveeffect differs from the piezoelectric effect. In contrast to thepiezoelectric effect, the piezoresistive effect only causes a change inelectrical resistance; it does not produce an electric potential. Thepiezoresistive effect of semiconductor materials can be several ordersof magnitude larger than the geometrical effect in metals and is presentin materials like germanium, polycrystalline silicon, amorphous silicon,silicon carbide, and single crystal silicon.

The proposed transparent photodetector is deemed transparent as only anegligible number of photons may be absorbed for the intensity of theoptical signal to be measured. Non-negligible optical losses in thestructure, which do not contribute to the detection mechanism, may occurfrom spurious scattering and material absorption. However, these opticallosses may be maintained at a negligible level as well by design of thetransparent photodetector.

Referring to the Figures in more detail, and particularly referring toFIG. 1, there is shown a first functional exemplary embodiment of atransparent photodetector 100. The transparent photodetector 100includes a waveguide 102 in close proximity 104 to a displaceablestructure 106. Connected to displaceable structure 106 is a siliconnanowire array 108.

A nanowire array may be defined as an array of nanowires with eachnanowire having a width and/or height on the order of 5 to 500nanometers, preferably below 150 nanometers, and a length of 5 to 5000nanometers, preferably below 500 nanometers.

The displaceable structure 106 may include a transparent section 110 anda conductive section 112. The transparent section 110 and conductivesection 112 are shown in FIG. 1 as separate, elements connected by arigid connection 114. However, it is within the scope of the presentinvention for the transparent section 110 to be adjacent to theconductive section 112 or even for the transparent section 110 andconductive section 112 to be combined into a single element having boththe functions of transparency and conductivity.

The silicon nanowire array 108 is electrically connected to theconductive section 112 of the transparent beam 106. The transparentphotodetector 100 may also include contacts 116 which also may makeelectrical contact with the silicon nanowire array 108.

The proximity 104 of the displaceable structure 106 and the waveguide102 is dependent on the wavelength of the light traveling in thewaveguide, the index of refraction of the transparent section 110 andother factors. The optimal proximity can be readily determined by aperson skilled in the art following the teachings of the presentinvention.

The transparent photodetector 100 may be fabricated on a suitablesubstrate 120 such as a semiconductor wafer. The displaceable structure106 and silicon nanowire array 108 are “suspended” with respect to thesubstrate 120. By suspended, it is meant that the displaceable structure106 and silicon nanowire array 108 do not make direct physical contactwith the underlying substrate 120. The waveguide 102 and contacts 116are not suspended which means that the waveguide 102 and contacts 116 domake direct physical contact with the underlying substrate 120.

In operation, a light 122 is caused to travel through the waveguide 102to transparent section 110 of the transparent beam 106. In thisexemplary embodiment, the traveling light 122 causes a force on thedisplaceable structure 106 which in turn causes a strain on the siliconnanowire array 108. Due to the enhanced piezoelectric effect in thesilicon nanowire array 108, the change in resistance of the siliconnanowire array 108 may be measured by applying a voltage and measuringthe output current 124. The output current 124 will be related to thepower of the traveling light 122 in the waveguide 102. Thus, thetransparent photodetector 100 may be used as a photodetector.

Referring now to FIG. 2, there is shown a second functional exemplaryembodiment of a transparent photodetector 200. The transparentphotodetector 200 includes a waveguide 202 in close, proximity 204 to adisplaceable structure 206. Connected to the displaceable structure 206is a silicon nanowire array 208.

The displaceable structure 206 may include a transparent section 210 anda conductive section 212. The transparent section 210 and conductivesection 212 are shown in FIG. 2 as separate elements connected by arigid connection 214. However, it is within the scope of the presentinvention for the transparent section 210 to be adjacent to theconductive section 212 or even for the transparent section 210 andconductive section 212 to be combined into a single element having boththe functions of transparency and conductivity.

The silicon nanowire array 208 is electrically connected to theconductive section 212 of the displaceable structure 206. Thetransparent photodetector 200 may also include contacts 216 which alsomay make electrical contact with the silicon nanowire array 208. It isnoted that in the exemplary transparent photodetector 200, the nanowirearray will be put into a compressive strain if the optical force isattractive. This will maximize the detectivity of the photodetector inthe case of an attractive optical force and a stronger silicon nanowirepiezoresistance in the compressive nanowire regime than in the tensilenanowire regime.

As noted above, the proximity 204 of the displaceable structure 206 andthe waveguide 202 is dependent on the wavelength of the light travelingin the waveguide, the index of refraction of the transparent section 210and other factors and can be readily determined by a person skilled inthe art following the teachings of the present invention.

The transparent photodetector 200 may be fabricated on a suitablesubstrate 220 such as a semiconductor wafer. The displaceable structure206 and silicon nanowire array 208 are suspended (as defined above) withrespect to the substrate 220. The waveguide 202 and contacts 216 are notsuspended.

In operation, a light 222 is caused to travel through the waveguide 202to transparent section 210 of the displaceable structure 206. In thisexemplary embodiment, the traveling light 222 causes an attractive forceon the displaceable structure 206 which in turn causes a strain on thesilicon nanowire array 208. Due to the enhanced piezoelectric effect inthe silicon nanowire array 208, the change in resistance of the siliconnanowire array 208 may be measured by applying a voltage and measuringthe output current 224. The output current 224 will be related to thepower of the traveling light 222 in the waveguide 202. Thus, thetransparent photodetector 200 may be used as a photodetector.

A first implementation of an exemplary embodiment of a transparentphotodetector 300 is illustrated in FIG. 3. Transparent photodetector300 may be implemented in a semiconductor on insulator (SOI) substrate320. SOI substrate 320 may include a top layer of silicon (also calledSOI layer) which may also be a silicon-germanium or silicon-carbon alloyand may include one or more dopants selected from the group consistingof germanium, arsenic, phosphorus, boron, antimony and carbon. Thetransparent photodetector 300 may be formed in the SOI layer. Underneaththe top SOI layer is an insulator such as oxide. The oxide insulatorlayer may also be called a buried oxide layer or BOX layer. On thebottom is the base substrate which is usually silicon and often referredto as the handle.

The transparent photodetector 300 includes a waveguide 302 in closeproximity 304 to a displaceable structure 306.

The displaceable structure 306 may include a transparent section 310 anda conductive section 312. The transparent section 310 and conductivesection 312 are shown in FIG. 3 as distinct although they may becombined into a single section if the material employed can be bothtransparent and conductive in the same area. The transparent section 310may have smooth transitions 328 to avoid scattering of light travelingthrough the waveguide 302. Connected to the displaceable structure 306and electrically connected to conductive section 312 are siliconnanowire arrays 308. Each nanowire may have at least one undoped orlow-doped segment along its length. Undoped and low doped silicon canshow larger piezoresistance than heavily doped silicon. The transparentsection 310 may be undoped to be highly transparent to light while theconductive section 312 is highly doped to offer a negligible electricalresistance when compared to the resistance of the nanowire array. Theconductive section 312 is shaded to indicate that it is doped. For astructure made of silicon or a silicon-germanium or silicon-carbonalloy, dopants may include boron, phosphorous, antimony and arsenic. Therequired doping of the low doping region (e.g., for the nanowires) andhigh doping region (e.g., for the conductive section) will be a functionof the exact dimensions of the device and the wavelength of light used.As a rule of thumb, if single-crystal silicon is used to define thetransparent photodetector 300, the highly doped region may exhibit aresistivity between 0.001 Ohm·cm and 1 Ohm·cm while the low-doped andundoped region may exhibit a resistivity of 0.1 Ohm·cm to 100 Ohm·cm.

The transparent photodetector 300 may also include contacts 316 whichalso may make electrical contact with the silicon nanowire arrays 308.The contacts 316 may also be doped similarly to the conductive section312. The contacts 316 are shaded to indicate that they may be doped. Itis noted that in the exemplary transparent photodetector 300, thesilicon nanowires are oriented such as an attractive optical force willcreate a compressive strain on the nanowire. This will maximize thedetectivity of the transparent photodetector if an attractive opticalforce is expected and piezoresistance of the silicon nanowires is largerin the compressive than in the tensile regime.

As noted above, the proximity 304 of the displaceable structure 306 andthe waveguide 302 is dependent on the wavelength of the light travelingin the waveguide, the index of refraction of the transparent section 310and other factors and can be readily determined by a person skilled inthe art following the teachings of the present invention.

The transparent photodetector 300 may be fabricated on an SOI substrate320 as indicated above. FIGS. 4A, 4B and 4C are various cross sectionsof transparent photodetector 300 as indicated by arrows 4A-4A, 4B-4B and4C-4C in FIG. 3. Referring first to FIG. 4A, this is a section throughthe conductive section 312, nanowire array 308, contacts 316,transparent section 310 and waveguide 302. FIG. 4B is a section throughthe conductive section 312, contacts 316, transparent section 310 andwaveguide 302 while FIG. 4C is a section through the conductive section312, transparent section 310 and waveguide 302. SOI substrate 320includes the base substrate 332, BOX layer 334 and SOI layer 336. Thedisplaceable section 306, including transparent section 310 andconductive section 312, and silicon nanowire arrays 308 may be suspendedby removing the BOX layer 334 underneath the SOI layer 336. Thedisplaceable section 306 and silicon nanowire arrays 308 are indirectlyconnected to the substrate 320 through contacts 316 which make directphysical contact with BOX layer 334. The waveguide 302 is also notsuspended and is directly physically connected to the substrate 320 atBOX layer 334.

In operation, a light 322 is caused to travel through the waveguide 302and interact with the transparent section 310 of the displaceablestructure 306. In this exemplary embodiment, the traveling light 322causes a force on the displaceable structure 306 which in turn causes astrain on the silicon nanowire arrays 308. Due to the enhancedpiezoelectric effect in the silicon nanowire arrays 308, the change inresistance of the silicon nanowire arrays 308 may be measured byapplying a voltage and measuring the output current. In this exemplaryembodiment, the current may be measured between two contacts 316 as itpasses through the nanowire arrays 308 and conductive section 312. Theoutput current will be related to the intensity of the traveling light322 in the waveguide 302. Thus, the transparent photodetector 300 may beused as a photodetector.

A second implementation of an exemplary embodiment of a transparentphotodetector 500 is illustrated in FIG. 5. Transparent photodetector500 may be implemented in a semiconductor on insulator (SOI) substrate520 as described previously with respect to FIG. 3.

The transparent photodetector 500 includes a waveguide 502 in closeproximity 504 to a displaceable structure 506.

The displaceable structure 506 may include a transparent section 510 andtwo conductive sections 512, one conductive section 512 on each side ofthe transparent section 510. The transparent section 510 and conductivesections 512 are shown in FIG. 5 as distinct elements although they areconnected and may be combined into a single section if the materialemployed can be both transparent and conductive in the same area. Thetransparent section 510 may have smooth transitions 526 to avoidscattering of light traveling through the waveguide 502. Connected tothe displaceable structure 506 and electrically connected to conductivesections 512 are silicon nanowire arrays 508. Each nanowire may have atleast one undoped or low-doped segment along its length to allow forlarge piezoresistance. The transparent section 510 may be undoped to behighly transparent to light while the conductive sections 512 are dopedto offer a negligible electrical resistance when compared to theresistance of the nanowire array. The conductive section is shaded toindicate that it may be doped.

The transparent photodetector 500 may also include contacts 516 whichalso may make electrical contact with the silicon nanowire arrays 508.The contacts 516 may also be doped. The contacts 516 are shaded toindicate that they may be doped.

As noted above, the proximity 504 of the displaceable structure 506 andthe waveguide 502 is dependent on the wavelength of the light travelingin the waveguide, the index of refraction of the transparent section 510and other factors and can be readily determined by a person skilled inthe art following the teachings of the present invention.

The transparent photodetector 500 may be fabricated on an SOI substrate520 as indicated above. The displaceable structure 506 and siliconnanowire arrays 508 may be suspended by removing the oxide layerunderneath the SOI layer similarly to that shown in FIGS. 4A-4C. Thedisplaceable structure. 506 and silicon nanowire arrays 508 areindirectly connected to the substrate 520 through contacts 516. Thewaveguide 502 is also not suspended and is directly physically connectedto the substrate 520.

In operation, a light 522 is caused to travel through the waveguide 502and interact with the transparent section 510 of the displaceablestructure 506. In this exemplary embodiment, the traveling light 522causes a force on the displaceable structure 506 which in turn causes astrain on the silicon nanowire arrays 508. Due to the enhancedpiezoelectric effect in the silicon nanowire arrays 508, the change inresistance of the silicon nanowire arrays 508 may be measured byapplying a voltage and measuring the output current. In this exemplaryembodiment, the current may be measured between two contacts of each setof contacts. The output current will be related to the power of thetraveling light 522 in the waveguide 502. Thus, the transparentphotodetector 500 may be used as a photodetector.

The exemplary embodiment of FIG. 5 mainly differs from the embodimentshown in FIG. 3 by the placement of the contacts. In both embodiments,the silicon nanowires are oriented such as an attractive optical forcewill result in a compressive strain on the nanowire. The preferredembodiment will depend on the required contact size, nanowire arraysize, and the required length of optical interaction between thewaveguide and the transparent section. These parameters can beestablished by a person of skill in the art based on the desireddetectivity of the photodetector, the desired bandwidth of thephotodetector and the fabrication process used. For situations where thecontacts or the nanowire arrays need to be larger than the desiredlength of optical interaction between the waveguide and the transparentsection, the embodiment of FIG. 5 may be preferred. For situations wherethe contacts or the nanowire arrays can be smaller than the desiredlength of optical interaction between the waveguide and the transparentsection, the embodiment of FIG. 3 may be preferred.

A third implementation of an exemplary embodiment of a transparentphotodetector 600 is illustrated in FIG. 6. This third implementation issimilar to the first implementation in FIG. 3 except that there may be asecond material utilized for the waveguide 602 and transparent section610.

Silicon is not transparent to all wavelengths that one may want todetect. Hence, a two-material implementation of the exemplaryembodiments may be of interest. The second material is showncross-hatched in FIG. 6. The second material may be used for thewaveguide 602 and the transparent section 610 of the displaceablestructure 606. Examples of a useful second material may be undopedsilicon nitride, silicon carbide, silicon dioxide, and aluminum oxide.Other materials are possible as well. In the third exemplary embodiment,the nanowire array 608 and the transparent section 610 may be on thesame plane to maximize nanowire strain detection by minimizing torquebending nanowires out of plane. If the second material is of a differentthickness than the silicon used for the nanowire array 608, anappropriate height offset may be planned to center the opto-mechanicalforce in the transparent section 610 on the plane defined by thenanowire array 608.

The contacts 616 and conductive section 612 are shaded to indicate thatthey may be doped. Each nanowire in the nanowire array 608 may have atleast one undoped or low-doped segment along its length to allow forlarge piezoresistance.

The transparent photodetector 600 may be fabricated on an SOI substrate620 and include a second material as indicated above. FIGS. 7A, 7B and7C are various cross sections of transparent photodetector 600 asindicated by arrows 7A-7A, 7B-7B and 7C-7C in FIG. 6. Referring first toFIG. 7A, this is a section through the conductive section 612, nanowirearray 608, contacts 616, transparent section 610 and waveguide 602. FIG.7B is a section through the conductive section 612, contacts 616,transparent section 610 and waveguide 602 while FIG. 7C is a sectionthrough the displaceable structure 606 and waveguide 602.

SOI substrate 620 includes the base substrate 632, BOX layer 634 and SOIlayer 636. The displaceable structure 606, including transparent section610 and conductive section 612, and silicon nanowire arrays 608 may besuspended by removing the BOX layer 634 underneath the SOI layer 636.The displaceable structure 606 and silicon nanowire arrays 608 areindirectly connected to the substrate 620 through contacts 616 whichmake direct physical contact with BOX layer 634. The waveguide 602 isalso not suspended and is directly physically connected to the substrate620 at BOX layer 634.

The waveguide 602 and transparent section 610 may be made of the secondmaterial and preferably are on the same plane as conductive section 612,nanowire array 608 and contacts 616 as can be seen in FIGS. 7A and 7B.At the side of the transparent beam 606, as shown in FIG. 7C, there maybe an overlap of the material (for example, silicon) 640 of the SOIlayer 636 and, the second material 642.

In operation, a light 622 is caused to travel through the waveguide 602and interact with the transparent section 610 of the displaceablestructure 606. In this exemplary embodiment, the traveling light 622causes a force on the displaceable structure 606 which in turn causes astrain on the silicon nanowire arrays 608. Due to the enhancedpiezoelectric effect in the silicon nanowire arrays 608, the change inresistance of the silicon nanowire arrays 608 may be measured byapplying a voltage and measuring the output current. In this exemplaryembodiment, the current may be measured between two contacts 616 as itpasses through the nanowire arrays 608 and conductive section 612. Theoutput current will be related to the intensity of the traveling light622 in the waveguide 602. Thus, the transparent photodetector 600 may beused as a photodetector.

The exemplary embodiments may be manufactured according to conventionalsemiconductor manufacturing practices following the teachings herein.

There are a number of design considerations for the exemplaryembodiments of the transparent photodetector. Among these designconsiderations are energy efficiency, detection bandwidth, nanowireorientation and typical dimensions.

Energy Efficiency:

Material loss, scattering loss and work performed by guided light areall considerations to minimize optical loss and hence maximize theenergy efficiency of the photodetector. A preferred embodiment willmaximize energy efficiency.

Material Loss

The materials used for the waveguide and for the transparent section ofthe displaceable structure that is exposed to a non-negligible amount oflight from the evanescent tail of the guided mode in the waveguide needto show low material absorption. Examples of such materials are silicon,silicon nitride, silicon oxide, silicon oxinitride, and silicon carbide.

Scattering Loss

Scattering by the transparent section of the displaceable structure ofthe optical power guided in the waveguide can be reduced to a negligiblelevel if the displaceable structure is smoothly (and preferablyadiabatically) approaching the waveguide on both edges. In addition, thetransparent section of the displaceable structure in the sensing region(region defined by sufficient proximity of the waveguide to thetransparent section of the displaceable structure for generation ofoptical forces) should preferably be of dimensions as not to support aguided mode by itself. If it does, significant amount of light may becoupled from the waveguide to the displaceable structure. It ispreferable for the displaceable structure to only introduce aperturbation to the mode of the waveguide without supporting a guidedmode by itself as this ensures that light will not be coupled to thedisplaceable structure. It is also preferable for the compositewaveguide in the sensing region (formed by the combination of thewaveguide and the displaceable structure) not to support higher ordermodes than the modes of the waveguide itself (far from the sensingregion). Imperfections in the structure may couple light to higher ordermodes which are then lost to radiation. The non-existence of these modescan prohibit this from happening and remove a possible source of loss.Of particular importance are higher modes of the polarization that thewaveguide is expected to carry. Not allowing these modes to appear inthe sensing region (combined waveguide) will contribute to energyefficiency.

Work Performed by Guided Light

To detect the optical signal, the guided light in the waveguide mustapply a force on the displaceable structure. This force is thentransmitted to the nanowires. This force must introduce a strain in thenanowires in order to modulate the output current via piezoresistance.Work is defined as the force times the displacement. To minimize theamount of work performed by the optical signal (energy lost todetection) one must ensure the displaceable structure is of as highstiffness as possible and the nanowires are as short as possible. Shortnanowires and stiff displaceable structure will ensure small beamdisplacement under the applied force by the optical signal. This willresult in a small amount of performed. Work and, thus, negligibleabsorption of photons.

Detection Bandwidth:

The detection bandwidth will be limited by the resonant frequency of themechanical resonator formed by the combination of the nanowires and thedisplaceable structure. The larger the stiffness and the smaller themass, the larger the resonant frequency is. Resonance enhancement is notrequired for signal detection. Hence, it will likely be preferable touse as large a nanowire-array stiffness as possible to set themechanical resonant frequency substantially above the modulationfrequency of the signal to be detected. This requirement is consistentwith the requirement of high stiffness for minimization of workperformed by guided light to optimize energy efficiency. In practice,the high stiffness can be achieved by using more nanowires in thenanowire array. The larger the stiffness, however, the larger the neededoptical force to induce a detectable strain in the nanowires. This canbe somewhat compensated with a larger sensing region.

Nanowire Orientation:

Piezoresistance in silicon nanowires is a function of the crystalorientation the nanowire surfaces. Any crystallographic orientation willwork but the preferred embodiment will use nanowires with large {110}surfaces. This can be obtained on a common <100> wafer by aligning thenanowires along or at 90 degrees to the <110> direction. The nanowiresidewalls will then be {110} oriented. If the nanowire cross-section istall and narrow, the piezoresistance of the nanowires will be dominatedby the {110} sidewalls which will result in a large detection signal.

Typical Dimensions:

Nanowire width and/or height: 5-500 nm

Nanowire length: 5-5000 nm

Bandwidth of the detected signal: <200 GHz.

The dimensions of the displaceable structure and waveguide depend on thematerials used. A typical silicon waveguide used for 1550 nm light wouldshow height and width in the 30-1000 nm range.

It will be apparent to those skilled in the art having regard to thisdisclosure that other modifications of the exemplary embodiments beyondthose embodiments specifically described here may be made withoutdeparting from the spirit of the invention. Accordingly, suchmodifications are considered within the scope of the invention aslimited solely by the appended claims.

What is claimed is:
 1. A transparent photodetector comprising: asubstrate; a waveguide for guiding a light on the substrate; adisplaceable structure that can move with respect to the substrate andis in proximity to the waveguide, the displaceable structure including atransparent section for interacting with the light guided by thewaveguide and a conductive section in rigid connection with thetransparent section; and a silicon nanowire array suspended with respectto the substrate and electrically connected to the conductive section ofthe displaceable structure, the silicon nanowire array comprising aplurality of silicon nanowires having piezoresistance; the substrate,waveguide, displaceable structure and silicon nanowire array forming thetransparent photodetector; wherein, in operation, light propagating inthe waveguide results in an optical force on the displaceable structurewhich further results in a strain on the nanowires to cause a change inelectrical resistance of the nanowires.
 2. The transparent photodetectorof claim 1 further comprising contacts affixed to the substrate andconnected to the silicon nanowire array.
 3. The transparentphotodetector of claim 1 wherein upon application of a voltage to thesilicon nanowire array, a current is measured which changes according toa change in the electrical resistance of the nanowires.
 4. Thetransparent photodetector of claim 1 wherein each nanowire has at leastone doped segment along its length.
 5. The transparent photodetector ofclaim 1 wherein the conductive section is proximate the transparentsection and the silicon nanowire array extends from the conductivesection and away from the transparent section.
 6. The transparentphotodetector of claim 1 wherein the conductive section is spaced fromthe transparent section and the silicon nanowire array and the contactsare between the transparent section and the conductive section.
 7. Thetransparent photodetector of claim 1 wherein the transparent section andwaveguide comprise a first material and the conductive section comprisesa second material, the first material being different from the secondmaterial.
 8. A transparent photodetector comprising: a silicon oninsulator (SOI) substrate comprising a semiconductor on insulator (SOI)layer, an oxide layer, and a base; a waveguide for guiding a lightformed in the SOI layer; a displaceable structure in proximity to thewaveguide, the displaceable structure formed in the SOI layer with theoxide layer removed from underneath the displaceable structure so thatthe displaceable structure is suspended with respect to the SOIsubstrate, the displaceable structure including a transparent sectionfor interacting with the light guided by the waveguide and a conductivesection in rigid connection with the transparent section; and a siliconnanowire array in the SOI layer suspended with respect to the base bythe removal of the oxide layer underneath the silicon nanowire array andelectrically connected to the conductive section of the displaceablestructure, the silicon nanowire array comprising a plurality of siliconnanowires having piezoresistance; the substrate, waveguide, displaceablestructure and silicon nanowire array forming the transparentphotodetector; wherein, in operation, a light source propagating throughthe waveguide results in an optical force on the displaceable structurewhich further results in a strain on the nanowires to cause a change inelectrical resistance of the nanowires.
 9. The transparent photodetectorof claim 8 wherein upon application of a voltage to the silicon nanowirearray, a current is measured which changes according to a change in theelectrical resistance of the nanowires.
 10. The transparentphotodetector of claim 8 wherein each nanowire has at least one dopedsegment along its length.
 11. The transparent photodetector of claim 8wherein the SOI layer further includes at least one dopant selected fromthe group consisting of germanium, arsenic, phosphorus, boron, antimonyand carbon.
 12. The transparent photodetector of claim 8 furthercomprising contacts in the SOI layer and connected to the siliconnanowire array, wherein the transparent section and conductive sectionform an enclosure with the contacts and silicon nanowire array withinthe enclosure.
 13. The transparent photodetector of claim 8 furthercomprising contacts in the SOI layer and connected to the siliconnanowire array, wherein the conductive section extends outwardly fromthe transparent section and the contacts and silicon nanowire array areproximate to the conductive section.
 14. The transparent photodetectorof claim 8 wherein the transparent section and waveguide comprise asecond material that is different from the material of the SOI layer.15. The transparent photodetector of claim 14 wherein the secondmaterial is selected from the group consisting of silicon nitride,silicon carbide, silicon dioxide, and aluminum oxide.
 16. Thetransparent photodetector of claim 8 further comprising contacts in theSOI layer and connected to the silicon nanowire array.
 17. Thetransparent photodetector of claim 16 wherein the transparent sectionand waveguide comprise a second material that is different from thematerial of the SOI layer.
 18. The transparent photodetector of claim 17wherein the second material is selected from the group consisting ofsilicon nitride, silicon carbide, silicon dioxide, and aluminum oxide.